Wish I'd found your post sooner.   I'm afraid this may come a bit late for you but maybe it could help someone else.

For huge objects that are at least partially round - like your eggs - I like to use a beach ball...a very large beach ball that is about 48" in diameter. (I bought mine on eBay, but you can find them at some novelty stores - search online for them.) 

For the narrow end, you could just use wadded up paper maybe in combination with a balloon as your frame or armature. Because you will be creating a sculture that you will want to cut open, a beachball is perfect. The things I use the beach ball for have openings and cut outs so I can deflate the ball and reuse it over and over.

If you you plan to make something "solid" you would either need to use something else for your form OR plan to leave an opening where you can deflate your ball and pull it out, then patch your opening. I've never tried this, but I feel certain that patching it wouldn't be difficult.

I have found some 36" balloons at novelty stores, but they tend to be kind of pricey. You wouldn't want to have to buy a large number of them for a large project.  I bought some from http://hornernovelty.com.  Their website leaves a lot to be desired but I live near them and don't have to rely on their site. If you don't live near them, their site provides a phone #. Or do a 'net search to find another novelty shop.

IMO, beachballs are the way to go. The really big ones still aren't cheap, but they can be used over and over again. Or when you are finished with them you can give them away to family and friends with pools! 

I hope this helps someone.